The development of “tight” oil and gas from within geological strata of low permeability has increased dramatically over the past few years due to advances in drilling and completion technologies that allow operators to create geological flow through the use of multi stage hydraulic fracturing operations in highly deviated or horizontal wells. This involves the construction of a completion arrangement that allows specific vertically deviated or horizontal sections of a well bore to be isolated and hydraulically fractured. Repeating this process along the length of the well bore in the target reservoir area ensures that controlled hydraulic fracturing of every potential flow-contributing zone is achieved.
Once the hydraulic fracturing has been completed, previously injected fluid as well as hydrocarbons present within the formation surrounding the wellbore are allowed to flow to the surface from deep within the subsurface strata. Currently the logging of these “extended reach” wells is not routinely carried out as it is expensive, time consuming and can be relatively inaccurate when attempted in a long horizontal well bore. However, operators wish to better understand the effectiveness of specific hydraulic fracture project design in extracting trapped oil or gas from reservoir zones in order that they can improve design for future development wells.
In the oil and gas exploration and production industry it is common to hydraulically fracture a hydrocarbon-containing rock formation, or reservoir, in order to allow the hydrocarbon to flow out of the rock through the rock fractures. Many methods of fracturing a rock formation and maintaining a fracture open for the flow of hydrocarbon are known and practised in the industry. It is also known in the art to trace the flow of fluids from a reservoir, including fluids flowing after a fracturing operation, using tracers. For example, European Patent Number 1991759 describes a method of monitoring the flow of fluid within or from a reservoir comprising the steps of inserting a solid non-radioactive tracer into the reservoir by means of a perforation tool, thereafter collecting a sample of fluid within or flowing from the reservoir and analysing said sample to determine the amount of said tracer contained in the sample. From the presence or absence of tracer in the sample, its amount and other parameters such as timing of the sample collection etc., information about the fluid flow within the reservoir may be gathered. U.S. Pat. No. 3,623,842 describes a method of determining fluid saturations in reservoirs by injecting at least two tracers having different partition coefficients between fluid phases (e.g. oil and water) into the formation and monitoring the appearance of the two tracers in the produced fluids. Radioactive tracers have been widely used for many years in well-monitoring applications. As an example, see U.S. Pat. No. 5,077,471, in which radioactive tracers are injected into a perforated well-bore, sealed and then monitored for decay to indicate the fluid flow from the formation. U.S. Pat. No. 4,755,469 describes the use of rare metal tracers for tracing oil and associated reservoir fluids by mixing an oil-dispersible rare metal salt with oil or an oil-like composition, injecting the dissolved tracer composition into a subterranean reservoir and analysing oil fluids produced from a different part of the reservoir for the presence of the rare metal to determine whether the oil mixed with the tracer has been produced from the reservoir.